1. Field of the Invention
The invention relates to capacitive measuring transducer apparatus, especially for small capacitive values, and more particularly to such apparatus with two bridge arms, each of which has at least one capacitor, and incorporating a differential amplifier connected on the input side with the bridge arms, each possibly having an amplifier, and the output of the differential amplifier is fed back to one of the bridge arms.
2. Prior Art
Such measuring transducer apparatus is disclosed in German OS No. 2 641 758. In case of this known measuring transducer, the two bridge arms, each of which has an amplifier, in the control circuit of which there is a capacitor or a parallel connection of capacitors and in the main circuits of which there always is a resistance combination for interconnecting terminal points of the bridge arms and an oscillator. The evaluation of capacitance changes of one or two of the capacitors disposed in different bridge arms takes place in the known case by way of the amplifiers acting as charge amplifiers, -which may be, for example, d.c. current operational amplifiers with a capacitive feedback, which may produce an output voltage, which is directly proportional to the change of the charge at the input connections. The differential amplifiers evaluate the output signals, to which a filter and a phase demodulator is series connected, the second input of which is connected with the oscillator, as well as a differential amplifier series connected to the phase demodulator, and a wave-band filter. The output voltage of the wave-band filter, at the same time, is fed back to a bridge arm via an analog multiplier, which precedes the latter.
The disadvantage of this known transducer lies in the fact that the individual parts of the circuit are very expensive and that the consequent costs are very high, to which the analog-multiplier also contributes considerably. Another disadvantage of this transducer, which operates according to the carrier-frequency method, also results from the fact that a sinusoidal generator is needed, in case somewhat higher demands are made, and this likewise results in a considerable expense.